Rename BR2_PREFER_STATIC_LIB to BR2_STATIC_LIBS

Since a while, the semantic of BR2_PREFER_STATIC_LIB has been changed
from "prefer static libraries when possible" to "use only static
libraries". The former semantic didn't make much sense, since the user
had absolutely no control/idea of which package would use static
libraries, and which packages would not. Therefore, for quite some
time, we have been starting to enforce that BR2_PREFER_STATIC_LIB
should really build everything with static libraries.

As a consequence, this patch renames BR2_PREFER_STATIC_LIB to
BR2_STATIC_LIBS, and adjust the Config.in option accordingly.

This also helps preparing the addition of other options to select
shared, shared+static or just static.

Note that we have verified that this commit can be reproduced by
simply doing a global rename of BR2_PREFER_STATIC_LIB to
BR2_STATIC_LIBS plus adding BR2_PREFER_STATIC_LIB to Config.in.legacy.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>